1. Technical Field
This invention relates to subsea well heads and particularly to the monitoring of pressure in the region between a production casing string and a relatively outer casing.
2. Related Art
Subsea wellheads support a multiplicity of casing strings which extend down into a well. These casing strings include a production casing string and a relatively outer casing string. These strings are usually within a larger casing known as the outer conductor. The annular region between the production string and the relatively outer string is usually called the B annulus. It is desirable to monitor the pressure in the B annulus and preferable for the monitoring to be performed by means of an associated production tree, i.e. a Xmas tree.
There exists a variety of designs such as those described in the documents GB-2376485-A and GB-2395736-A for this general purpose. These and other existing designs are however quite complex. In particular they include penetrations of the wellhead housing and conduits either through this housing or external thereto to a region above any casing hanger. There exists a need for a simpler yet reliable design.
It is known (for example from U.S. Pat. No. 5,366,017) to provide communication from the B annulus to the tree by way of a passage through a production casing hanger. However, the passage needs blocking during the stages of drilling and the installation of the casing hanger and it is the object of the present invention to provide an assembly which allows the selective closure and opening of the passage by means of an easily operated sealing adaptor.